The present invention relates to a machine for the security printing of security paper, especially banknotes, comprising a paper sheet feed device, a device for transporting the paper and a unit for applying security images supported by at least one ribbon which is brought into contact with the paper such that these images are applied at determined points on the paper corresponding to the printing on the security paper, the application unit comprising an applicator cylinder fitted with pads arranged on the periphery of the applicator cylinder in register with said determined points, cooperating with an impression cylinder, between which cylinders the paper passes, said ribbon being mounted between two reels--for paying out the ribbon, and for winding up the ribbon--the ribbon passing parallel to the path of the paper.
It is already known practice to create security zones on security paper and in particular on banknotes, by applying images in the form of a film so as to make these papers difficult to falsify, particularly to reproduce by the use of photocopiers, the reproduction quality of which is ever increasing. These images are often optically variable images comprising either a kinegram or a hologram which have the property of changing appearance depending on the angle from which they are viewed. These images may be applied either hot or cold. In the European patent application No 0 625 466 A1, the content of which is incorporated by reference, by the same applicant, there is described an apparatus that allows such images to be applied cold using a two-part adhesive, the first part of the adhesive being applied at the desired point on the notes which have been partially or completely printed on a sheet and it is dried by passing the sheet past a catalyst. Thereafter, using an applicator cylinder and a press cylinder, the images, which have the second part of the adhesive, are applied to the previously chosen points, simply by the action of pressure.
In the abovementioned machine, the ribbon travels between two cassettes along a generatrix of the applicator cylinder. The applicator cylinder is fitted with a number of pairs of these cassettes to allow several images to be applied to several points of a sheet in a single pass.
There is also another type of apparatus which comprises a unit for applying an optically variable image borne by a ribbon of which there is one for each impression column and which is paid out between a paying-out reel and a winding-up reel. The path between the two reels comprises, apart from the guiding and tensioning rolls and rollers, a cylinder which has circumferential grooves, interrupted by bridges, said ribbon passing along said grooves. The sheets to be processed pass between said cylinder and press cylinders and image transfer occurs each time a bridge runs over the press cylinder. The applicator cylinder is heated to activate the adhesive borne by the image. In order that the adhesive should be heated sufficiently to obtain the desired effect, contact between the bridge of the applicator cylinder and the images must last for a certain length of time, which means that the dimensions of the applicator cylinder and those of the press rolls must always be tailored to the size of sheet and of the security papers partially or completely printed on said sheets. The ribbon carrying the images passes continuously and in synchronism with the sheets to which the images are to be applied. This apparatus produced by the applicant is satisfactory.
However, the obligation to maintain contact between the applicator cylinder and the image to be transferred for a period of time means that each time the paper format is changed, fundamental modifications have to be made to the applicator cylinder and to the press cylinder. Furthermore, the press cylinder used to keep the sheets in contact with the applicator cylinder prior to the transferring of the image, crushes the sheet along the entire length at the two ends of the sheet in the direction of travel of the paper. Specifically, for the sheet to remain in contact with the applicator cylinder, two small rollers press the sheet against the applicator cylinder near to the ends of the sheet before the sheet receives the images so that the bridge of the applicator cylinder is in close contact with the ribbon and allows the adhesive on the images to be applied to be brought up to the desired temperature, and afterwards, the sheet passes over a press cylinder which is the same width as the sheet, this allowing the images to be applied to the paper.
Thus, the pressure exerted by the two rollers which, from the outset, ensures that the sheet is pressed firmly against the applicator cylinder, leaves undesirable marks on the sheet.
The object of the present invention is to propose a machine similar to the one described earlier, but which does not have the aforementioned disadvantages.